Hi,
best way is to have all voip devices in own vlan(s) and a policy at all switches to priorise traffic with this source vlan(s) with DSCP 46.
With this solution I prefere the option "qos no phones" to untrust all others prio and use only my own one.
But also there are a lot of other solutions (f.e. with trust) - but I try always to use the vlan depending prio.
regards
Silvio